1. Field of the Invention
This invention relates to methods and an apparatus for measuring gas flow in a pipeline. More particularly, it relates to a tandem rotor turbine meter and field calibrator module which comprises a second independent metering rotor placed behind or downstream of an existing or main independent metering rotor and separated from it by flow conditioning stator vanes. There are two operation modes for this apparatus: the first being continuous operation as a tandem rotor turbine meter and the second being periodic use as a field calibrator module.
2. Prior Art
Following World War II, the construction of high pressure, interstate natural gas pipelines increased the need for accurate and reliable measurement devices for measuring large volumes of gas flow in a pipeline. The shortcomings of the traditional orifice meter were overcome with the development of gas turbine meters. Turbine meters provide great rangeability, compact size, and simplified maintenance when compared to alternative methods of large volume measurement. Single and double-rotor turbine meters as well as dual turbine meter systems are currently commercially available, each of which have drawbacks peculiar to their operation. For example, single-rotor turbine meters are not well adapted to provide accurate measurement in gas flow streams with non-uniform velocity profiles or with mechanical degradation of the rotor. Existing double-rotor turbine meters either do not have their rotors close enough together such that temperature and pressure corrections are required due to different flow conditions at each of the rotors; or in the case where they are in close proximity of each other, an example of which is shown in FIG. 1, the first rotor 1 affects the output of the second rotor 2 and thus two sets of independent output are not available. The two rotors 1 and 2 can be contained within the same meter body and module housings 3 and 3'. Rotor 1 is preceded by stator vanes 4. Between rotors 1 and 2 is a thrust balancing plate 5 whose function is to separate the rotors thereby balancing the axial thrust load on rotor 1. The force of the flow impinging upon the blades of rotor 1 causes a downstream force. The flow of gas over the hub of the thrust balancing plate creates a dynamic back pressure force counteracting the axial thrust.
In the prior art system, the rotor 1 operates in a manner similar to a single rotor meter because it actually measures the gas that passes through the line. The gas actually turns the rotor and then, through a mechanical gearing, that motion is transmitted out through a magnetic coupling to an output coupling to which the instrumentation is mounted. The output coupling is calibrated to represent a predetermined number of cubic feet per revolution thus generating a reading. In addition to rotor 1, there is a second rotor 2 whose function is to sense any change in direction of the gas velocity vector exiting from the blades of rotor 1. These two rotors therefore are fluidly coupled. The term fluid as used by those skilled in the art and as used throughout this specification shall mean both liquids and gas. If the first rotor is affected by fluid friction or mechanical friction, as in the running gear itself, the second or "sensing" rotor can sense this by the gas velocity vector angle change at which the flow leaves the tip of the first rotor's trailing edge and will change speed accordingly. The pulse output from both the rotors, taken together (via complex mathematical equations), indicate the true flow of fluid in the pipeline. For example, if a swirl of fluid is coming down the pipeline, it may hit the blades of the first rotor at such an angle that the rotor will give a reading that may indicate a higher flow rate than the actual flow rate in the pipeline. The rotational speed of the rotors (rpms) is a function of the actual vector velocity of the fluid. If the fluid enters the meter at an angle, the first rotor will be misled. However, the second rotor is designed to distinguish between the velocity of the fluid flowing through the pipeline and the velocity of fluid flow which comes off the trailing edges of the blades of the first rotor. Using the two rotor rotational speeds, a computer program calculates a ratio and adjusts the output at all times. Specific examples of various types of turbine meters are described in the following patents.
An example of an insertion type turbine meter is disclosed in U.S. Pat. No. 4,566,307 ('307 patent), entitled "Pipeline Flow Measurement Proving System". By way of background, an insertion turbine meter is a mechanical device used to measure the flow of gas or liquid through a pipe of known internal diameter. It has a small turbine rotor mounted on the end of a long stem. The diameter of the rotor is significantly smaller than the internal diameter of the pipe. The rotor and stem are inserted through a port in the side of the pipe. The rotor is positioned at the approximate center of the pipe and oriented in-line with the pipe axis. The speed of the gas flow causes the turbine rotor to spin. The rotational speed of the turbine rotor is proportional to the local velocity of the gas. Typically, an electronic pickup or pulse is used to sense the speed of the rotor or to count its revolutions. This output is then factored by a multiplier based on the internal diameter of the pipe to obtain a reading of total flow volume. The two meters used in the '307 patent are separated by a distance of about 25 feet in an attempt to eliminate the influence of one meter on the other. Such a separation requires pressure and temperature correction between the two rotors for output totalization and accuracy performance. In addition, insertion turbines are not as accurate as full pipeline turbine meters. Insertion turbine meters are not as accurate because they do not measure the entire flow passing through the pipeline. Therefore, in order to obtain the correct reading of average flow velocity, the orientation and position within the pipe is critical. This also limits the useful range of the insertion turbine meter since the location of the average flow velocity does change between laminar and turbulent flows. Also, the presence of the insertion turbine meter disturbs the flow profile. Due to their small size, the insertion turbine meter can not incorporate flow conditioners ahead of the rotor. Thus, their accuracy is affected by flow disturbances. Their calibration accuracy is also affected by the actual internal pipe diameter. Their typical accuracy is .+-.2% or more as compared to .+-.1% error for full flow turbine meters.
An example of double rotor apparatus is disclosed in U.S. Pat. No. 2,859,616, entitled "Mass Flow Meter", in which the rotors are not in close proximity of one another and thus likewise require pressure and temperature correction between the two rotors for output totalization and accuracy performance. It does disclose the summary of two electronic signals from each rotor, however, one output does not check the other.
U.S. Pat. No. 4,286,471, entitled "Constant Accuracy Turbine Meter" by Lee et al discloses a turbine meter in which a sensing rotor downstream from the metering rotor senses changes in the exit angle of the fluid leaving the metering rotor, the output from the sensing rotor being combined with the output from the metering rotor to produce a corrected output indicative of the flow through the meter. The output from the sensing rotor is utilized through a closed loop feedback system to modify the operation of the metering rotor in accordance with variations in the exit angle of the fluid leaving the metering rotor. The two rotors must rotate in the same direction.
The metering system disclosed in U.S. Pat. No. 4,305,281, entitled "Self-Correcting Self-Checking Turbine Meter" by Lee et al is very similar to that disclosed in U.S. Pat. No. 4,286,471 in that it likewise discloses a turbine meter in which a sensing rotor downstream from the metering rotor senses changes in the exit angle of the fluid leaving the metering rotor (thus the rotors are fluid coupled), the output from the sensing rotor being combined with the output from the metering rotor to produce a corrected output indicative of the flow through the meter. The output from the sensing rotor and the output from the metering rotor may be compared to provide an indication of deviation from performance at calibration. Additionally, the two rotors must rotate in the same direction and are fluid coupled.
Thus all gas measuring devices known to date fail to provide a system in which two independent rotors are located close enough together to alleviate the need for temperature and pressure corrections resulting from differences in flow conditions at each rotor, while at the same time being isolated from the effects of each other.
Accordingly, it would be desirable to have two independent rotors, in close proximity of each other, housed in the same meter body, but isolated from the effects of each other. This arrangement would eliminate the need for pressure and temperature corrections due to differences in flow conditions at each rotor. It would also be desirable to have direct comparison of two rotor outputs thereby allowing continuous checking of the condition of each rotor. It is further desirable to have a system which allows accuracy testing of turbine meters in service over a broad flow range, and at actual operating conditions of fluid pressure, temperature, density, and fluid chemical composition.